1. Field of the Invention
The present invention relates to automatic parking brakes and, more particularly, to a sequencing valve that prevents release of an automatic parking brake until the braking system is fully charged and, in some cases, a subsequent brake application is made.
2. Description of the Related Art
In order to avoid having to manually apply the brake on each rail car in a train to accomplish a parking brake function, automatic parking brakes having been developed that automatically retain the brake cylinder in a fully applied position if the brake pipe pressure is exhausted below a predetermined threshold. For example, a locking mechanism responsive to brake pipe pressure may be included within a body mounted brake cylinder to prevent the push rod from returning to the brakes released position and a locking mechanism response to brake pipe pressure may be positioned on the exterior of a truck mounted brake cylinder to prevent the piston from returning to the brakes releases position. Because the development of brake pipe pressure releases the locking mechanism when the threshold is exceeded, it is possible that the parking brake will be released, thereby allowing the rail car to roll, before a normal brake application can be applied to hold the train in place. Accordingly, there is a need for a system that will prevent the automatic parking brake from releasing the rail car into a potential rolling situation until a normal brake application is possible or actually applied.